The present invention relates to magnetic hard disk drives. More specifically, the present invention relates to a method of testing the stroke and the frequency response of a micro-actuator used in a hard disk drive.
Hard disk drives are common information storage devices essentially consisting of a series of rotatable disks, or other magnetic storage mediums, that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers, are typically carried by and embedded in a slider body that is held in a close relative position over discrete data tracks formed on a disk to permit a read or write operation to be carried out. In order to properly position the transducer with respect to the disk surface, an air bearing surface (ABS) formed on the slider body experiences a fluid air flow that provides sufficient lift force to “fly” the slider and transducer above the disk data tracks. The high speed rotation of a magnetic disk generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body which enables the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface through this self-actuating air bearing.
Some of the major objectives in ABS designs are to fly the slider and its accompanying transducer as close as possible to the surface of the rotating disk, and to uniformly maintain that constant close distance regardless of variable flying conditions. The height or separation gap between the air bearing slider and the spinning magnetic disk is commonly defined as the flying height. In general, the mounted transducer or read/write element flies only approximately a few micro-inches above the surface of the rotating disk. The flying height of the slider is viewed as one of the most critical parameters affecting the magnetic disk reading and recording capabilities of a mounted read/write element. A relatively small flying height allows the transducer to achieve greater resolution between different data bit locations on the disk surface, thus improving data density and storage capacity. With the increasing popularity of lightweight and compact notebook type computers that utilize relatively small yet powerful disk drives, the need for a progressively lower flying height has continually grown.
FIG. 1 illustrates a hard disk drive design typical in the art. Hard disk drives 100 are common information storage devices consisting essentially of a series of rotatable disks 104 that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers, are typically carried by and embedded in a slider body 110 that is held in a close relative position over discrete data tracks formed on a disk to permit a read or write operation to be carried out. The slider is held above the disks by a suspension. The suspension has a load beam and flexure allowing for movement in a direction perpendicular to the disk. The suspension is rotated around a pivot by a voice coil motor to provide coarse position adjustments. A micro-actuator couples the slider to the end of the suspension and allows fine position adjustments to be made.
In order to properly position the transducer with respect to the disk surface, an air bearing surface (ABS) formed on the slider body 110 experiences a fluid air flow that provides sufficient lift force to “fly” the slider 110 (and transducer) above the disk data tracks. The high speed rotation of a magnetic disk 104 generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The airflow cooperates with the ABS of the slider body 110 which enables the slider to fly above the spinning disk. In effect, the suspended slider 110 is physically separated from the disk surface 104 through this self-actuating air bearing. The ABS of a slider 110 is generally configured on the slider surface facing the rotating disk 104, and greatly influences its ability to fly over the disk under various conditions. To control the in-plane motion of the slider, especially to access various data tracks on the disk surface, the head suspension assembly (HSA) typically incorporates a primary actuator. The primary actuator may be a voice coil located at the end opposite the read/write head. Due to the large inertia of the HSA, the primary actuator has limited bandwidth. Vibration of the suspension makes it difficult to control the read/write head position from a distance. The primary actuator along has difficulty achieving the speed and accuracy of position required.
Advanced disk drive design incorporates a secondary actuator, or micro-actuator, between the read/write head and the pivotal axis of the HSA. The stroke, or distance of displacement in relation to the voltage applied, of these micro-actuators is typically in the order of 1 μm. FIG. 2a illustrates a micro-actuator with a U-shaped ceramic frame configuration 201. The frame 201 is made of, for example, Zirconia. The frame 201 has two arms 202 opposite a base 203. A slider 204 is held by the two arms 202 at the end opposite the base 203. A strip of piezoelectric material 205 is attached to each arm 202. A bonding pad 206 allows the slider 204 to be electronically connected to a controller. FIG. 2b illustrates the micro-actuator as attached to an actuator suspension flexure 207 and load beam 208. The micro-actuator can be coupled to a suspension tongue 209. Traces 210, coupled along the suspension flexure 207, connect the strips of piezoelectric material 205 to a set of connection pads 211. Voltages applied to the connection pads 211 cause the strips 205 to contract and expand, moving the placement of the slider 204. The suspension flexure 207 can be attached to a base plate 212 with a hole 213 for mounting on a pivot via a suspension hinge 214. A tooling hole 215 facilitates handling of the suspension during manufacture and a suspension hole 216 lightens the weight of the suspension.
The read/write head is routinely tested before shipment. Typically a read/write head is flown over a spinning disk connected to a dynamic parametric (DP) tester, and a sequence of read/write activity is performed. A DP test may be conducted when the read/write head is assembled in either a head gimbal assembly (HGA), a HSA, or a head-disk assembly.
The DP test may include testing the stroke of the micro-actuator at various input voltages. One method for testing the stroke is shown in FIG. 3. The quasi-static stroke may be measured by writing a first concentric track 310 and a second concentric track 320 at distinct constant input voltages and then obtaining a “track profile” over the above tracks by reading at successively increasing or decreasing radii. The track profile may compare the radial position of the read head 330 with the read back signal 340. The input voltages may be no input, maximum input, or negative maximum input. Each peak of the track profile indicates a center of a track. The stroke may be calculated using the distance 350 between the adjacent peaks.
The DP test may also include testing for frequency response. The frequency response compares the stroke to the input frequency. As shown in FIG. 4, the frequency response may be measured by, at each input frequency, first erasing a band of disk surfaces, then applying a predetermined alternating input voltage at a desired frequency while writing over this erased band for approximately one revolution of the disk. After removing the input voltage, map the written signal by reading at successively increasing, or decreasing radii, while recording the amplitude of the read-back signal as a function of both radius and angular position. A sinusoidal curve 410 is mathematically fit to the track readings which represents locations with peak amplitude of the read-back signal for each track. The amplitude 420 of this sinusoidal curve is the stroke at the given frequency.
These methods are slow. For each desired frequency the disk surface must be erased and re-written. At each frequency the amount of data required to map the written signal is also large, because the map of FIG. 4 is two-dimensional instead of one-dimensional. Other methods for measuring micro-actuator frequency response include optical and electrical tests. In the optical test, a laser beam is directed at the read/write head or its vicinity. The reflected light is collected and analyzed for the velocity or displacement of the reflecting surface. This method requires expensive equipment and delicate alignment. The electrical test is possible for micro-actuators with more than one active element. The input voltage is applied on some but not all of the elements. Those elements not receiving the input voltage are driven mechanically by other elements, thus generating a small output voltage from which stroke may be derived. However, this mode of excitation differs from the mode in application, in which all elements receive input voltage. Thus the frequency response pertains to a vibration mode other than the mode of actual concern. Furthermore, when an HSA has multiple read/write heads, accessing only the head of interest without interference of the other heads may become difficult.